This invention relates to temperature sensors which may be used for measuring temperatures within the bore of a tube or especially adapted for use in dry out detection apparatus. It is known that there is a certain condition which arises when a local region of a heat emitting rod cooled by convection is no longer wetted by liquid but is blanketed with a vapour film. This is a critical condition known as dry out and it is important to plant designers to have data relating to this condition so that its occurence is minimised. This is especially so where the heat emitting body is a nuclear fuel rod and experiments have been set up which simulate a fuel rod by the use of an electrical resistance heated rod of similar exterior shape. These rods are termed heaters and special heating electrical resistances have been used to give a predetermined, non uniform, variation in heat axially along the rod as well as uniform heat emission, or so called uniform axial heat flux distribution. Besides having the required heating resistor, the rod also incorporates a dry out detection means from which knowledge of the dry out temperatures and the preferential region or regions where the dry out temperatures occur, may be obtained. In the case where the rod is non-uniformly heated e.g. where the heat flux along the rod follows a cosine distribution the actual location of dry out region is very much dependent upon rig conditions and cannot be readily predetermined to better than say about .+-. 30 cms. It is therefore necesssary to provide means for sensing temperature changes over several meters of heated length. One method employed to do this involves monitoring the resistance change in resistance thermometers by means of Wheatstone bridges in each of which two of the resistance sensors form arms of a bridge. Each bridge require regular re-balancing and with a large number of bridges, this task becomes impractical.